1. Field of the Invention
This invention relates to an improvement in the operation of AC motors. More particularly, this invention uses a tuned vibration absorber to suppress the 120 Hz pole noise in AC motors.
2. Description of Related Art
AC synchronous motors are found in a variety of drive applications and are preferable to DC motors when simplicity, reliability and low cost are the design criteria. However, DC motors are typically selected because AC motors have a relatively large 120 Hz velocity error that results from the 60 Hz commutation. This velocity error is approximately three percent of the steady state velocity. Therefore, light-lens xerographic platforms, which are tolerable to 120 Hz electrical pole noise, use an AC motor. Single image black and tri-level systems can also use AC motors. However, laser printing is much more sensitive to velocity errors and can tolerate a velocity error of only 0.25%.
DC motors are usually selected for a high performance apparatus, e.g., digital printers and photoreceptive drive belts, because standard AC motors fail to deliver the required motion quality to xerographic printers. However, the DC motors require feedback operation because of the strict motion quality requirements. Other alternatives to AC motors are the DC servo-controlled motor, which is more costly, or separate motor systems for each driven part of the printer. The DC motor is the typical motor chosen for photoreceptor drives and multiple image color systems because motion quality requirements for digital systems are much more stringent.
A typical 60 Hz synchronous AC motor has 4 poles respectively located at quarter turns in the rotation direction. As the rotor of the motor rotates at 30 rev/s, an oscillation motion of 120 Hz results. Similarly, a typical 50 Hz synchronous AC motor has an oscillation motion of 100 Hz. One way to reduce this oscillation motion is to add inertia to the rotor to reduce the vibration. However, by increasing the inertia, the motor is harder to start and stop. Theoretically, an appropriately sized flywheel could provide adequate attenuation, but it would be so large that there would be significant increases in coast and start-up times. These increases may be so large that start-up may be prevented altogether. The prior art fails to provide sufficient attenuation with minimal addition to system inertia.